There are a number of solder bump forming methods according to the prior art. According to a plating method, metal is deposited on electrode pads of a microelectronic substrate through plating to form bumps. In another method typically referred to as a stencil printing method, solder paste, typically including flux, is printed onto electrode pads of a microelectronic substrate through a patterned stencil, and then, after stencil removal, the device is heated to melt the solder to form bumps therefrom. Stencil printing is sometimes used by the prior art to deposit differing amounts of solder paste onto the electrode pads of a substrate as a function of a size of the solder resist opening on each of the electrode pads. Thus, the prior art sometimes uses a stencil mask exhibiting larger openings in registration with larger solder resist openings of the substrate, and smaller openings in registration with larger solder resist openings of the substrate, in this way depositing a corresponding amount of solder paste onto each electrode pad. However, it has been observed that the stencil printing method is not suited for high density interconnection structures, typically leading to missing bump rates, bump voiding, and bump height variation, thus negatively affecting die attachment yields.
Different techniques have been introduced to address ever growing demands for pitch and solder resist openings (SRO) size reductions, such as pitches of about 160 microns and SRO sizes of about 80 to 90 microns. One such method involves the placement of micro balls or micro spheres of solder onto the electrode pads of a microelectronic substrate. According to an attachment mounting micro ball placement method, solder balls are sucked into a jig by vacuum suction and the solder balls then mounted onto flux-coated electrode pads of a microelectronic substrate. Another micro ball placement method involves the use of a stencil mask. According to the latter methods, solder balls of a uniform size are dispensed onto a ball alignment plate or stencil mask including holes therein in registration with electrode pads of a microelectronic substrate. A squeegee brush is then used to disperse the balls and press them into the mask holes. The electrode pads include flux thereon, which allows the balls to adhere thereto. The stencil mask is removed after ball placement. The solder balls are then heated and melted to form bumps.
The prior art poses problems however, among others where bumps are to be provided on a substrate having electrode pads and/or solder resist openings of differing sizes. A micro ball placement method, such as the attachment mounting method or the stencil mask method described above, typically results in solder bumps exhibiting significant bump height variations from bump to bump depending on the size of the electrode pad and/or solder resist opening used.
The prior art fails to provide a reliable method of providing solder bumps on electrode pads and/or solder resist openings of differing sizes on a microelectronic substrate.
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